The present invention generally relates to protective coatings for components exposed to high temperatures within a chemically and thermally hostile environment. More particularly, this invention is directed to a method and apparatus for controlling the deposition of a diffusion coating on internal passages of a component, such as an air-cooled gas turbine engine component, so as to promote a more uniform coating thickness that is better capable of protecting the internal passages from oxidation and corrosion.
The operating environment within a gas turbine engine is both thermally and chemically hostile. As higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-base superalloys. Nonetheless, when used to form certain components of the turbine, combustor, and augmentor sections of a gas turbine engine, superalloys are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties.
A common solution is to protect the surfaces of such components with an environmental coating, i.e., a coating that is resistant to environmental attack, typically in the form of oxidation and hot corrosion. Coatings that have found wide use for this purpose include diffusion coatings, such as diffusion aluminides and chromides, and overlay coatings such as MCrAlX (where M is nickel, cobalt and/or iron and X is X is yttrium or a rare earth or reactive element). During high temperature exposure in air, these coatings form a protective oxide scale that inhibits oxidation of the coating and the underlying substrate. Diffusion aluminide coatings are particularly useful for providing environmental protection to components equipped with internal cooling passages, such as high pressure turbine blades, because aluminides are able to provide environmental protection on the cooling passages without significantly reducing their cross-sections, which otherwise would lead to insufficient cooling flow and shortened life of the component.
Diffusion coating processes, such as pack cementation, vapor phase (gas phase) aluminiding (VPA), and chemical vapor deposition (CVD), generally entail contacting the surface to be coated with a reactive vapor that contains the desired material to be deposited, often aluminum. In the case of vapor phase aluminiding, a source of aluminum (for example, CO2Al5) and a halide salt activator (for example, AlF3, NH4F, KF, NH4Cl) are placed in a container along with the components to be coated, and the container is then placed in a retort that provides a gas shield for the container. The retort is heated to cause the activator to react with the aluminum source and form a volatile aluminum halide, which then reacts at the component surfaces to form the diffusion coating. An outermost zone of the coating is often termed an additive layer that contains the environmentally-resistant intermetallic phase MAI, where M is iron, nickel or cobalt, depending on the substrate material. A diffusion zone (DZ) forms within the substrate beneath the additive layer, and contains various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. During high temperature exposure in air, the additive layer forms the desired alumina scale that inhibits oxidation of the diffusion coating and the underlying substrate. Typical thicknesses for diffusion aluminide coatings are about 30 to 75 micrometers for the additive layer and about 25 to 50 micrometers for the diffusion zone.
Achieving a suitable diffusion coating thickness, uniformity, and internal/external thickness ratio for an air-cooled component can be difficult, particularly for turbine blades with complex external geometries and cooling passage designs. To control the amount of coating deposited on the internal passages of a turbine blade, the reactive aluminum halide vapor is typically forced through the internal passages. For example, the reactive vapors can be introduced into the blade through its root and flow through the internal passages before exiting through cooling holes at the component surface, for example, film cooling or blade tip holes in the airfoil surfaces of the blade. Alternatively, the coating vapors can be forced to enter through the cooling holes and exit at the blade root.
The reactivity of the coating vapor decreases as it flows through the blade and deposits aluminum, resulting in a thinner coating (and potentially no coating) near the exit points. If the coating operation is extended to increase the coating thickness at the exit points, the coating can become excessively thick in the vicinity where the vapors entered the blade and on the external surfaces. Because excessive coating thickness can adversely impact airflow and reduce the strength of the underlying alloy, a blade with this condition is subject to rejection at the manufacturing level. As such, controlling the relative thickness distribution inside a blade would be beneficial to achieving the required protection in service without incurring a reduction in material properties due to overly thick coatings in high stress areas, such as the blade shank.